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Dpi? Spi? Ppi? Lpi?

Discussion in 'Help Team' started by Fen, Jul 30, 2011.

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  1. Fen

    Fen Well-Known Member

    DPI? SPI? PPI? LPI? It's a nightmare isn't it?

    Hard/software manufacturers, magazines and websites haven't made it any easier by mixing and matching the terms as if they were interchangeable, they're not.

    DPI - Dots Per Inch:
    In this discussion is best used when describing the output from a desktop printer.

    SPI - Samples Per Inch:
    Rarely used unfortunately but it would be better if used more often when describing the number of input samples per inch of the source material made by a scanner.

    PPI - Pixels Per Inch:
    Best used when describing the digital output of a source, be it a camera or software.

    LPI - Lines Per Inch: Although often also referred to as DPI
    Describes the halftone* frequency of a printing press used for newspapers, magazines and the like, often the intended clients of photo libraries.

    *Halftone being the small dots that make up the image, when looked at closely, in a newspaper, magazine or book.

    Now…
    Firstly it would be better to ask 'Where does 300ppi come in to it?' :)

    In relation to submissions to libraries, magazines etc for final output on a halftone press it comes in to it in so far as… stay with me… the accepted rule is that the ppi resolution of an image should be double the lpi halftone frequency of the final printed output at 1:1.

    Now… newspapers usually have an lpi halftone frequency of 85-100lpi, magazines 150-185lpi. The 'rule' (which can be broken) says that an image appearing in a magazine that uses a halftone frequency of 150lpi should have a resolution of 300ppi at 1:1.

    In other words the 'rule' says that at 1:1 the input resolution should be double the output resolution (for halftone printing)**.

    **The obvious issue being that when submitting images it is unlikely you will know at what size they will finally be used. It is for this reason that libraries usually ask for a minimum file size. For example a file in the region of 4960x3508 pixels or 48MB RGB would allow for an A3 DPS (Double Page Spread) halftoned reproduction using a 150lpi halftone frequency at 300ppi and as such cover most of the bases.


    Don't forget ppi not dpi in this case :)
    Yes. If you were to resize a 1"x1" digital file with a resolution of 7200ppi with constrained proportions and no resampling to 300ppi nothing would change except the image dimensions.

    The 1"x1" 7200ppi image would now be a 24"x24" 300ppi image. The number of pixels it contains would remain the same.

    -- Thank you to (Group Art Editor) Mark Jacobs for the explanation --
     
    Michelle Nelson likes this.
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